Identify necessary outputs from the input data db

[tsmbland]

We need to think about what data we will need to retrieve from the database and what format it will take. This will probably be xarrays which are identical to objects already existing in MUSE (see readers/csv.py). We should identify what these objects are, including a list of dimensions, variables, and data types.

It's probably better to look into this sooner rather than later as it might inform some design choices for the database.

To do:

• Update docstrings for the functions in csv.py to give more info about the xarray objects returned (dimensions, variables, data types)
• Add tests in test_readers.py to check the outputs of these functions
• Identify a suitable subsection of the schema to begin implementing a minimal database, and the expected outputs from this database. I've suggested the commodities section (blue tables in the schema) as an option, but open to other ideas

[tsmbland]

Closed by #346